Android resources should be added to the corresponding drawable-XXX folders inside the App_Resources\Android folder in your app:

The content of this directory will be copied inside the platforms\android\res when the app is prepared by the NativeScript CLI. More information about how to use drawable resources in Android can be found here.

IOS resources should be added inside the App_Resources\ios folder in your app. You can use @1x, @2x and @3x suffixes to target devices with a specific screen scale. Here is a list of devices for each scale factor: